1. Field of the IventionN
The present invention relates to an optical communication network having a collision detection function of transmitted optical signals.
2. Prior Art of the Invention
As one multiplex method in an optical communication network utilizing an optical fiber, a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) method conventionally used an electrical communication network is employed.
FIG. 7 is a schematic block diagram of a communication interface unit of a terminal device employing the CSMA/CD method in the optical communication network (to be referred to as an optical CSMA/CD method hereinafter). The operation of the terminal will be described below with reference to FIG. 7.
As shown in FIG. 7, a terminal 39 connected to an optical fiber 10 via a optical coupler 21 and a communication interface transmits a signal to be transmitted onto the optical fiber 10, and sends it to another terminal on the network. In this case, a transmission operation based on the optical CSMA/CD method is performed in the following protocol.
The terminal 39 which requests transmission outputs a control signal indicating a data transmission request to a communication controller 34. Upon reception of this signal, the communication controller 34 causes a carrier detector 25 to check if a signal from another terminal is transmitted onto the optical fiber 10. If it is detected that no signal from another terminal exists on the optical fiber 10, the communication controller 34 outputs, to the terminal 39, a control signal indicating that it is possible to start transmission. A transmission signal output from the terminal 39 in response to this control signal is encoded by an encoder 31, and the encoded signal is then converted into an optical signal by an optical transmitter 24. The optical signal is transmitted onto the optical fiber 10 via a demultiplexing/multiplexing element 22 and the optical coupler 21. In this case, the signal sent from the encoder 31 is input to the carrier detector 25, a collision detector 35 and a decoder 30 together with a signal received by an optical receiver 23, thereby the communication controller 34 monitors whether or not the transmission signal is normally transmitted.
With the above-mentioned protocol, data transmission from the terminal 39 is started. At the same time, another terminal connected to the optical fiber 10 may start transmission. In order to cope with such a case, the communication controller 34 causes the optical receiver 23 and the collision detector 35 to monitor whether or not a signal from another terminal is transmitted onto the optical fiber 10 while the terminal 39 is transmitting data.
When the collision detector 35 detects a collision between a signal transmitted from its own terminal 39 and a signal from another terminal, the communication controller 34 informs to the terminal 39 using a control signal indicating that the collision occurred, and sends a cancel signal indicating that the transmitting signal is invalidated, onto the optical fiber 10 via the encoder 31. After an elapse of a randomly set time, the communication controller 34 re-executes the above-mentioned transmission protocol, and restarts the interrupted data transmission.
However, the prior art suffers from the following problems.
As shown in FIG. 7, on a transmission path using the optical fiber 10, light reflection is inevitably caused by a connecting portion, e.g., a connecting portion between the optical coupler 21 and the optical fiber 10, a connecting portion between optical fibers 10, or the like. In particular, when a bus-type network is employed as a network pattern, so that terminals can be relatively freely connected/disconnected, it is difficult to eliminate reflection at the connecting portions.
On the transmission path of the optical fiber 10, when a connecting portion which reflects light is at a distance of x meters from the optical coupler 21, light transmitted from the optical coupler 21 onto the optical fiber 10 is reflected and returned after (2x/C.sub.f) sec (where C.sub.f is the light propagation speed in the optical fiber).
If a delay time (2x/C.sub.f) of a reflected wave is sufficiently shorter than a time corresponding to one bit of a signal to be transmitted, no problem is posed. However, if the delay time is almost equal to or longer than the time corresponding to one bit, the collision detector 35 cannot identify whether a detected signal is a signal which is transmitted from its own terminal and reflected, or a signal from another terminal. As a result, the collision detector 35 may erroneously detect a collision of signals.
For example, if a transmission speed is set to be 100 Mbps, the one-bit length of the signal is 10 nsec. In this case, if the distance x is about 2m or more, a collision is erroneously detected.